Look at all the pieces of the puzzle....
Clue 1: With the pixel density of the the 7D, only the top Lglass lenses are able to outresolve the sensor.
Clue 2: Canon has recently been upgrading it's high end lenses to outresolve said density.
That they realize that a high megapixel camera will be useless without the appropriate lenses to take advantage of it.... That this has been in the planning for a long time....
I'm not sure I understand this comment about needing better lenses for the high MP FF.
As you've correctly pointed out, the 40 MP FF would have about the same pixel density as the current 7D (which I have) and I haven't seen any reason to think that there is a problem with the "old" line of lenses and the 7D. My 500 f4L IS USM (no II) is just fine with the 7D.
If its a good lens, its a good lens. If its a bad lens, be it poor resolution or barrel distortion, it will show up the same unless you're printing larger, won't it?
See my visual examples of the difference between good glass (EF 100-400mm f/4.5-5.6 L IS) vs. excellent glass (EF 500mm f/4 L II IS + 1.4x TC) in the quote below of one of my previous posts on another thread. There are significant differences between good glass and excellent glass when the 7D (or for that matter any one of Canon's 18mp cameras) are involved.
To put some images behind my claims. Below are two photos of House Finches. One is the normal red morph, the other an orange morph. Same bird, otherwise, same size (maybe a slight size benefit to the orange morph) with the same amount of base detail...feathers, beak, eye. Both of these were shot at pretty much the same distance (around 7 feet...red morph maybe a few inches farther), ISO, and aperture, although the red one was up in a tree so my focal plane was shifted a bit, thus slightly blurring the top of its head and the back of its right wing. The body feathers and beaks are in focus on both birds. Both birds were positioned within the same rough area of the lens...slightly off center towards the upper left corner. Both full-scene images below are cropped to roughly the same area (few pixels difference in width and height).
Both photos shot with my 7D, ISO 400, f/6.3, in my backyard. The red morph was shot with my EF 100-400mm f/4.5-5.6 L IS lens with a full stop of additional light at twice the shutter speed (1/1600s, which should be an IQ advantage!) The orange morph was shot with a rented EF 500mm f/4 L IS II. Both lenses had AFMA adjustments for this body.
Here are the full images, scaled down to 900 pixels. Even at this level, you can see the difference in quality between the two photos can be seen. The orange morph is sharper and clearer (probably thanks to better microcontrast.)
At 100% crop (1:1 zoom, PIXEL PEEPING for all you pixel peepers!), the difference in IQ is beyond clear. The 100-400mm lens produces far softer results (even ignoring the slightly out of focus crest on the red morph). This kind of softness is what I've come to expect from the 100-400mm lens at less than f/8, and beyond f/8 diffraction again softens the image. (There is roughly the same amount of noise in both photos. It is more apparent in the red morph due to the increased lens softness, which blurs detail but does NOT blur noise. Clear, sharp detail tends to trump noise. The background in the red morph also provides a greater area of <= 18% gray tone, where noise becomes most apparent...the orange morph has a greater area of pixels > 18% tone.)
Scaled down to web size, the red morph photo is good enough. Most people won't notice the slight softness. From a print standpoint, I probably would not print the red morph photo, however the orange morph photo is definitely printable. It is not only printable, it could also easily be blown up two, maybe three times larger, and still be high quality, even higher quality than the red morph photo printed at original size!